The present invention relates to a liquid crystal driving system using dynamic driving means.
Conventionally, it is known that liquid crystal characteristics deteriorate when a liquid crystal is driven by the constant application of a specific voltage in the same direction between two electrodes. In one technique used to prevent such deterioration, an AC-converted signal capable of varying the voltage polarity of the electrodes is applied to each frame in which one scanning cycle of the liquid crystal matrix is completed. However, this technique has the disadvantageous effect that, as the duty factor increases, an uneven contrast is generated between the identical ON and OFF picture elements, resulting in a significantly degraded display. In extreme cases, when comparing a segment line using the highest frequency and a segment line using the lowest frequency, a nearly equivalent contrast may undesirably be generated between the OFF picture elements of one segment line and the ON picture elements of the other segment line. For example, as shown in FIG. 10, the conventional 16.times.10 dot matrix liquid crystal display unit is provided with segment-side electrodes 1 through 16 and common-side electrodes A through J. The third column of the segment-side electrodes has a specific pattern and uses the highest frequency to switch alternate dots ON and OFF on every other line, whereas the fourth column of the segment-side electrodes uses the lowest frequency to switch all dots OFF. Line F of the sixth row of common-side electrodes has dots "a" and "b" which both remain OFF. Although both of these dots should be provided with identical OFF contrast, for the reasons described below, these dots differ in contrast and cause the display to become uneven, eventually degrading the overall display quality. FIG. 11 shows the waveforms of the drive signals used with a 16.times.10 dot matrix liquid crystal display unit, in which the driving method uses a duty of 1/10 and a bias of 1/3. The factors affecting the contrast and display quality are listed below.
(A) AC-converted signal M, whose polarity is inverted in each frame period TM. PA1 (B) Common signal CF, which drives line F in the sixth row of the common-side electrodes. PA1 (C) Segment signal S4, which drives the fourth column of the segment-side electrodes. PA1 (D) Signal Vb (CF-S4), which is applied to dot "b". PA1 (E) Segment signal S3, which drives the third column of the segment-side electrodes. PA1 (F) Signal Va (CF-S3) which is applied to dot "a".
Ideally, the effective values of signals Va and Vb as well as the OFF-contrast of dots "a" and "b" should be identical. However, in actuality, the waveforms are subjected to distortion caused by the resistance of the electrodes, the capacitance of the liquid crystals themselves, and the driving capacity of the liquid crystal driving circuit. This waveform distortion eventually causes a difference between signals Va and Vb. In this example, since the waveforms of signal Vb are less affected by distortion than signal Va, signal Vb has a greater effective value than Va. As a result, dot "b" generates a higher OFF-contrast than does dot "a". The same is true of the ON-contrast effect. This phenomenon is even more noticeable when a higher duty is used.